Electrohydrodynamic jet (E-jet)printing uses electric-field induced fluid flows through micro capillary nozzles to cause a fine stream of drops to be formed and ejected. Typically, these electric fields are created by establishing a potential difference between the nozzle carrying the ink (the print head) and the receiving print substrate. A DC voltage is applied to the nozzle, causing the mobile ions in the ink to gather near the surface. This causes the meniscus at the nozzle tip to change into a conical shape, typically referred to as a Taylor cone, due to the tangential stress and attraction to the substrate. This is an unstable state that eventually results in a periodic drop release from the apex of the cone.
Other ways of forming a continuous stream of drops include drop-on-demand ink-jet printing using thermal and piezo-excitation and continuous inkjet printing using electrostatic or air deflection to direct the drops to a gutter or the receiver selectively. Among these, traditional ink jet printing systems are limited to low viscosity inks (say, less than 5 cP). Electrohydrodynamic jet printing has demonstrated superior resolution, printing of micron and sub-micron scale drops using a wide variety of inks. E-jet has been shown to work with fluids as high as 90 cP which makes it possible to use a much greater range of printing inks.
These developments are still inadequate, however, to open up a much greater range of applications to ink jet printing such as 3-D printing fluids and functional fluids whose viscosity is in the range of 15-100 cP or greater because the pulsed conditions lack sufficient control to print uniform drops (both in size and period between drops) in the kHz range.
Improved control of the drop formation of an E-jet system can be achieved by using a pulsed voltage on the capillary nozzle. In particular, control of the timing of the drop formation and the regularity of the drop size can be achieved by using a voltage profile, shown in FIG. 1 that has successive pulses at a fixed periodicity. This technique has proven effective with higher viscosity fluids in the sub kHz range, but not with consistent control of drop period and size in the kHz range. FIG. 2, adapted from High-speed and drop-on-demand printing with a pulsed electrohydrodynamic jet, S Mishra et al., J. Micromech. Microeng. 20 (2010), pages 1-8, shows the 1 kHz printing of Norland Optical Adhesive 74. The magnified edge of the image has an indeterminate period and drops size. It is unclear from the image what the true fundamental period that corresponds to the 1 kHz input is. The larger drops have a larger period. It does not appear as if the drops moved and merged on the surface, rather they are a result of missing every other print. This resulted in twice as larger of a drop at half the printing rate.
What is needed is a way to precisely control drop size and period with a range of higher viscosity fluids in the kHz range. Additionally, enhanced process controls to independently regulate process outputs such as drop size and delivery frequency also is desired.